The deleterious effect of polyvalent metal ions, e.g., iron, calcium and magnesium, on the viscosity, rheology and flocculant activity of water soluble anionic polymers is known. The ferrous form of iron is the most notorious as it can exert its adverse effect at levels as low as about 2 parts per million (ppm)or lower. At levels of about 10-15 ppm of ferrous iron, about five times the amount of an anionic polymer is needed to achieve the same flocculant activity as is needed in the absence of the iron. In some instances, it may take more than 5 times the amount. An anionic polymer flocculant can almost completely lose its activity in a period of 6-8 hours in the presence of 15-25 ppm of ferrous iron. Higher levels of ferric iron are needed before an adverse effect on the anionic polymer is observed. Calcium and magnesium generally require levels exceeding 100 ppm before they affect the flocculant activity of an anionic polymer. The anionic polymers may be exposed to the polyvalent metal ions from a variety of sources including the make-up water in which the polymer is dissolved or the aqueous environment in which the flocculant is used.
To overcome this problem, sodium hydroxide has been added to the make-up water used to prepare the anionic polymer flocculant. However, this technique is not generally utilized because of the difficulties associated with obtaining the appropriate pH to cause the precipitation of the iron without causing the substantial hydrolysis of the polymer.